Inducing a protective antibody (Ab) response to HIV Envelope (Env) is a major strategy for vaccine-mediated prevention. The occurrence of HIV broadly neutralizing antibodies (bNAb) during HIV infection demonstrates the capability of the human immune response; however, defining the developmental queues of bNAb development and recapitulating their induction and persistence by vaccination remains elusive. Our work has begun to define the B cell features associated with HIV bNAb and protection in humans and non-human primates. The primary focus for studying HIV-specific B cell responses thus far has been IgG and IgA, however relatively minimal investigation has been focused on the contribution of IgM memory to effective HIV-specific responses. Within the human IgM memory population, subsets have been identified in blood including marginal-zone-like and B1-like B cells, however the inter-relationships between these functionally and phenotypically overlapping populations remains unclear. B cells at mucosal sites include abundant IgM producing antibody-secreting cells (ASC), and half of the mucosal IgA is derived from B-1 B cells. Our previous work has demonstrated the unique characteristics of HIV Env-specific IgM memory B cells in humans and its association with the incidence of bNAbs in HIV-infected subjects and broader Env reactivity in HIV vaccinees. Numerous features of IgM memory suggest that with adequate engagement it could be a valuable contributor to an effective B cell response to HIV. These features include their rapid response, unique and polyreactive immunoglobulin repertoire, neutralizing activity, expansive mucosal distribution, strong complement activation, and enhanced antigen presentation abilities. Additionally, IgM antibodies have been shown to contribute to neutralizing Ab responses against many other viruses. The ability of IgM memory B cells to differentiate upon antigen-stimulation into IgG (and IgA) memory and ASC populations may contribute to qualitatively distinct Ab responses. Mechanisms to induce robust IgM memory responses to protein antigens remain poorly defined and current strategies to induce protective humoral response to HIV may have limited ability to induce beneficial Env-specific IgM memory. Our central hypothesis is that immunization strategies that induce robust Env-specific IgM memory responses will enhance protection from HIV infection. This hypothesis will be tested by the following specific aims: 1) to optimize strategies for the induction of HIV Env-specific IgM memory utilizing in vivo mouse experiments, 2) to determine the protective activity of HIV Env-specific IgM responses in rhesus macaques through an immunogenicity and challenge experiment and 3) to characterize human HIV Env-specific IgM memory through in-depth phenotypic and functional profiling. This project will significantly advance our insight into preventing HIV transmission and the mechanisms that control the development of protective humoral responses to HIV.